The present invention relates to closed-loop control systems and more especially to control devices for use in such systems.
Closed-loop control systems in their simpler forms generally comprise a sensor for measuring a parameter of interest, a reference signal source which produces a signal corresponding to the set-point or desired value of the measured parameter, a comparator, usually a differential amplifier, which produces a signal proportional to the difference between the measured and set-point signals and a controller which functions to exert a controlling influence on the process being controlled in such manner as to reduce the difference between the set-point and measured signals to zero or to a suitably low value. It should, of course, be noted that the parameter which is measured need not be the same as the parameter on which the controller exerts its influence, although there must be some inter-relation between the measured and controlled parameters in order to achieve the closed-loop situation.
One of the problems with control systems of the above type is that because the rate at which energy can be transferred between energy stores is finite, time delays arise around the control loop. This, combined with the fact that the input and output of a closed-loop control system are interactive gives rise to a response for the system involving one or more real or complex exponential terms. A consequence of this is that the response of a closed-loop control system to a step change in the set-point is generally exponentially variant with time, the response frequently being of the form of a damped sinusoid. Although this form of response can be useful or tolerated in some applications and control systems can be optimised to give a small following error in response to step changes in the set-point, nevertheless, there are some processes where a more linear change with respect to time is desirable. Frequently it is desired also to provide for a controlled rate of change with respect to time and to facilitate adjustability of the rate of change.